1. Field of the Invention
The present invention relates to a fixing belt used in image forming apparatuses such as electrophotographic copiers and printers for fixing a toner image, formed on a recording medium, to the recording medium; a fixing unit employing the fixing belt; and an image forming apparatus employing the fixing unit. More particularly, the invention relates to an arrangement wherein the fixing belt with a lubricant applied to its inside surface is driven into rotation and is pressed against the recording medium formed with the toner image thereon so as to fix the toner image to the recoding medium, and is characterized by preventing the lubricant applied to the inside surface of the fixing belt from leaking out of the belt.
2. Description of the Related Art
In the image forming apparatuses such as electrophotographic copiers and printers, it has been a conventional practice to fix the toner image, formed on the recording medium, to the recording medium by means of the fixing unit.
Conventionally, various types of fixing units have been used for fixing the toner image, formed on the recording medium, to the recording medium.
One of such fixing units has the following arrangement as shown in FIG. 1. An endless fixing belt 21 is pressed against a rotating pressure roller 23 by means of a pressing member 22 disposed at place inside of an inside surface of the fixing belt. In conjunction with the rotation of the pressure roller 23, the fixing belt 21 is driven into rotation by way of a force of friction between the pressure roller 23 and the fixing belt 21. On the other hand, a heat roller 24 containing therein a heating element 24a is disposed at place inside of the inside surface of the fixing belt, so that the fixing belt 21 is heated by the heat roller 24.
In the fixing unit, a sheet-like recording medium 9 formed with a toner image t thereon is oriented in a manner to present the toner image t to the fixing belt 21 and is guided into space between the fixing belt 21 and the pressure roller 23. The recording medium 9 is heated and pressurized as sandwiched between the fixing belt 21 and the pressure roller 23, whereby the toner image t is fixed to the recording medium 9.
In the fixing unit employing such a fixing belt 21, the following measure is generally taken to ensure that the above fixing belt 21 is properly driven into rotation in conjunction with the rotation of the pressure roller 23. That is, a lubricant, such as a grease, is previously applied to the inside surface of the fixing belt 21 so as to reduce frictional resistance between the fixing belt 21 and the pressing member 22.
Even in the case where the lubricant such as a grease is applied to the inside surface of the fixing belt 21, the following problem is encountered. The lubricant is decreased in quantity because the pressure exerted by the pressing member 22 causes the lubricant to leak from the inside surface of the fixing belt 21. Consequently, frictional resistance between the fixing belt 21 and the pressing member 22 is increased so that the fixing belt 21 is not smoothly driven into rotation. Hence, slippage between the pressure roller 23 and the fixing belt 21 occurs to disturb the toner image t to be fixed to the recording medium 9 or to cause the recording medium 9 to be jammed between the fixing belt 21 and the pressure roller 23.
More recently, therefore, there have been proposed a fixing unit wherein the inside surface of the aforesaid fixing belt is roughened so as to retain the lubricant such as a grease (Japanese Unexamined Patent Publication No. 2001-341143), and a fixing unit wherein the inside surface of the fixing belt is formed with recesses for retaining the lubricant such as a grease (Japanese Unexamined Patent Publication No. 2002-25759).
However, even in the case where the inside surface of the fixing belt is roughened or formed with the recesses for retaining the lubricant such as a grease, the lubricant applied to the inside surface of the fixing belt gradually leaks from the widthwise opposite ends of the fixing belt because of the pressure exerted by the pressing member. While the fixing belt is used over an extended period of time, therefore, the slippage between the pressure roller and the fixing belt still occurs to disturb the toner image to be fixed to the recording medium or to cause the recording medium to be jammed between the fixing belt and the pressure roller. Thus, the fixing units are unable to provide consistent image fixation over an extended period of time.